1. Field of the Invention
This invention relates to a process and an apparatus for supercritical water oxidation and, in particular, to such a process and apparatus useful for the destruction of organic waste materials.
2. Description of the Prior Art
It is known that the fluid and solvating properties of water change dramatically at its thermodynamic "critical point"--i.e., at a temperature of about 706.degree. F. and a pressure of about 3204 pounds per square inch ("psi"). In particular, water above its critical point is a single-phase fluid which is completely miscible with oxygen and most organic compounds. At supercritical water conditions, mass transfer limitations which limit the usefulness of oxidation processes in subcritical water are eliminated and the solubility of inorganic salts drops to the parts per million range. With the addition of oxygen, efficient destruction of organic compounds by oxidation is achieved, while inorganic substances can be separated and withdrawn for disposal. Specifically, organic compounds may be oxidized in supercritical water to produce carbon dioxide and water. Following the oxidation, inorganic materials may be removed as dry solids or precipitated as salts and removed as a brine. A significant advantage of such a process is the extremely short residence time required, on the order of 5-20 seconds depending on process temperature, for destruction of organic compounds.
Supercritical water oxidation--defined as a process for oxidizing organic waste compounds in supercritical water--has emerged as a potentially environmentally attractive technique for the safe and effective treatment of toxic organic wastewaters and sludges. However, while organic compounds are completely miscible in supercritical water, inorganic salts present in the aqueous waste materials, or generated during the oxidation reaction, are essentially insoluble and may be deposited as solids on reactor and other system surfaces, which may lead to plugging and low operating efficiencies. In addition to that solids deposition problem, corrosion of reactor and system surfaces has also occurred in supercritical water oxidation processes. Due to these problems, supercritical water oxidation has not been successfully commercialized for the treatment of organic wastes and sludges.
Various attempts have been made to rectify these problems. For instance, U.S. Pat. No. 4,822,497 discloses a reactor designed so that the supercritical temperature process stream is transferred to a cooler zone in the same vessel at high pressure to form a brine which facilitates the removal of solids. More recently, U.S. Pat. No. 5,252,224 discloses a supercritical water oxidation process in which the reaction mixture is passed through the reactor at a velocity sufficient to prevent settling of a substantial portion of the solid particles from the reaction mixture.
There is still a need for a supercritical water oxidation process and system which avoids the above-mentioned significant problems of solids deposition and corrosion.